PLC Programming - How Do The Different Languages of IEC 61131-3 Compare?

I don't think that PLC programming is very difficult. And like any computer, you need to tell it what it is that you want it to do. Without you, it will do nothing. It will lay there like a plastic brick. It needs to be programmed and that is the job of the PLC Programmer.

Back in 1993 and later revised in 2003 the IEC complied the standard know as 61131. IEC 61131-3 is the portion of the standards document that deals with programming of PLCs. The main reason of IEC 61131-3 was standardization of existing PLC languages.

The five languages in the IEC Standard 61131-3 are:

  1. Instruction List (IL): A low-level language very similar to assembly language. I have used it when programming old Phillips PLC. The code is compact and suitable for small projects. It not very powerful. The other languages are easier to use and document. It has been years since I have used it and do not miss it.
  2. Structured Text (ST): A high level language structured like Pascal. Users trained in high level text languages would be comfortable with ST.
  3. Ladder Diagrams (LD): Also commonly known as Ladder Logic. Modeled after the electrical wiring of contacts and relays used to create logic. It made the transition from Relay logic (using actually electrical relays) to the PLC easier. It falls short when tasks become complex. Not the best for modular PLC programming. This language will look very familiar to Electricians.
  4. Function Block Diagram (FBD): The blocks contain procedures or functions to act on the input "wires" and output the result. It lends itself readily to standardizing, modulating, and maintaining programs. I prefer this over LD, and many are coming to realize its advantages. Ladder Diagrams will die a slow death.
  5. Sequential Function Chart (SFC): This graphic language is great for concurrent parallel sequential operations. It is self documenting. It very useful pulling together in a flow chart form the other PLC programming elements such as function blocks (FB) or structured text (ST). Its format shows overall program flow very well making it faster and easier to understand what the program is doing. Easy to identify the section of interest for troubleshooting and program improvement. By the way...a common belief is that you would be able to quickly convert between the above languages. This is false. Although they have common elements they also have differences that preclude the possibility.

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